Study of configuration differentia and highly efficient deep-red thermally activated delayed fluorescent organic light-emitting diodes based on phenanthro[4,5-fgh]quinoxaline derivatives

Article abstract of DOI:10.1039/d1tc01562a

The development of efficient thermally activated delayed fluorescent (TADF) materials with a deep-red emission (emission wavelength beyond 620 nm) remains a great challenge in the field of organic light-emitting diodes (OLEDs). In this work, a series of s

Full text of DOI:10.1039/d1tc01562a

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Materials Chemistry C
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Study of configuration differentia and highly
efficient deep-red thermally activated delayed
fluorescent organic light-emitting diodes based
on phenanthro[4,5-fgh]quinoxaline derivatives†
Cite this: J. Mater. Chem. C, 2021,
9, 7392
Anqi Shang,‡a Tong Lu,‡a Hui Liu,a Chunya Du,a Futong Liu,a Dongyan Jiang,a
a
Jiarui Min,b Haiquan Zhang*b and Ping Lu
*
The development of efficient thermally activated delayed fluorescent (TADF) materials with a deep-red
emission (emission wavelength beyond 620 nm) remains a great challenge in the field of organic light-
emitting diodes (OLEDs). In this work, a series of structural isomers are successfully obtained by
attaching an electron-donor triphenylamine (TPA) group to the various positions of two types of planar
acceptor units, PQP and PQCN. Interestingly, the trans-PyCNTPA and trans-PyPTPA all exhibit much
higher photoluminescence quantum yields (FPLs) (87% and 81%) than their respective cis-isomers,
cis-PyCNTPA (19%) and cis-PyPTPA (12%). The theoretical calculations and dynamic studies indicate the
effective suppression of the nonradiative decay processes in trans-isomers. The trans-PyCNTPA and
cis-PyCNTPA show relatively small DEST and efficient reverse intersystem crossing to ensure the complete
utilization of triplet excitons, indicating that the TADF characteristics are regulated by the co-acceptor of the
CN unit owing to its strong electron-withdrawing property. As a result, a deep-red TADF-OLED based on
trans-PyCNTPA is achieved with a maximum external quantum efficiency (EQE) of 15.5% at 668 nm, CIE
coordinates of (0.66, 0.35) and stable electroluminescence spectra at different voltages. The results present
an efficient method to develop efficient red TADF emitters by isomer optimization strategy.
Received 4th April 2021,
Accepted 15th May 2021
DOI: 10.1039/d1tc01562a
rsc.li/materials-c
energy gap (DEST) is the basic parameter to achieve effective
TADF effect through RISC. DEST, which directly restricts the
Introduction
Materials with emission wavelength in a deep-red/near infrared magnitude of the intersystem crossover rate, is expressed as
(NIR) region are of particular importance owing to their wide follows:31,32
applications in bioimaging,1–5 chemosensing,6–8 night-vision-
devices,9,10 information-secured displays,11–13 and organic
light-emitting diodes (OLEDs),14–24 etc. So far, the deep-red/
NIR fluorescent materials with good performance in OLEDs still
remain rare. The high photoluminescence quantum yield (FPL)
and exciton utilization efficiency (FEUE) need to be fulfilled
simultaneously to acquire satisfactory external quantum efficiency
(EQE) in OLEDs according to the spin statistics theory.25 In this
case, the thermally activated delayed fluorescence (TADF) emitter
becomes the recent concern mainly because of the promising full
exciton utilization through the efficient reverse intersystem
crossing (RISC) process.26–30 The very small singlet–triplet
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1
DEST ¼ 2J ¼ 2fhfe
f2f1
(1)
rhꢀe
where J represents the singlet–triplet electron exchange energy,
and |rh-e| is the distance between the positive charge center and
the negative charge center. Therefore, the small DEST can be
realized by creating a twisted donor–acceptor (D–A) molecular
structure with sufficient separation between the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO).33,34
Generally, FPL is determined by the following equations:35
Kr
FPL
¼
(2)
Kr þ Knr
Knr p aeꢀbDE
a State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Qianjin Street No. 2699, Changchun, 130012, P. R. China. E-mail: lup@jlu.edu.cn
b State Key Laboratory of Metastable Materials Science and Technology, Yanshan
University, Hebei Street No. 438, Qinhuangdao, 066004, P. R. China.
E-mail: cnhqzhang@ysu.edu.cn
(3)
where Kr is the rate constant of radiative decay, Knr is the
nonradiative decay rate constant, a and b are proportionality
constants, and DE is the energy gap between the zero-vibrational
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1tc01562a
‡ Dr Anqi Shang and Dr Tong Lu contributed equally to this work.
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levels of S1 and the higher-vibrational levels of S0 states. There- into the PQP unit as the co-acceptor to increase the electron-
fore, with the reduction of DE in the deep-red/NIR zone, withdrawing strength and red-shift the emission spectra, which
the vibrational overlap between S0 and S1 will manifest an might contribute to obtain a small DEST and promote the RISC
exponential increase of substantial energy loss via non-radiative process according to the formulas 2 and 3. In addition, trans-
36,37
processes, leading to a relatively low FPL
.
According to the PyCNDPA and trans-PyPDPA using diphenylamine (DPA) as the
Franck–Condon transition principle and Fermi’s golden rule, donor are also synthesized as the model compounds to inves-
small orbital overlaps in TADF materials will inevitably result in tigate the function of an extra benzene ring in trans-PyCNTPA
small transition dipole moments (M) and thus lead to a small and trans-PyPTPA. All the six compounds show twisted patterns
oscillator strength (f) between the S1 and S0 states, which is not caused by the relatively big steric hindrance of TPA and DPA,
38
conducive to obtain a high FPL
.
Consequently, it is still a which diminish aggregation caused quenching (ACQ) effect to
formidable challenge to achieve highly efficient deep-red/NIR some extent. The introduction of bridging benzene in trans-
39–47
fluorescent materials and OLEDs with high FPL and FEUE
.
PyCNTPA and trans-PyPTPA could balance the relationship
Pyrene is an interesting representative of polyaromatic between the small DEST and high FPL for red light-emitting
hydrocarbons (PAH) with a very simple structure of four fused materials. The positional change is found to significantly affect
benzene rings. Combination of rigidity and planarity in mole- the photophysical properties and the trans-isomers all exhibit
cular structure entitles pyrene with favorable p-electron flow, nearly 4-folded higher FPLs as compared with their cis-counterparts.
high FPL and controllable excimer formation, etc.48 Function- The trans-PyCNTPA, trans-PyCNDPA and cis-PyCNTPA show obvious
ality of the pyrene skeleton to improve its electronic properties TADF features suggesting that the introduction of the CN group is
has been a hot topic in an optoelectronic field in the past years, crucial to obtain a small DEST to realize the RISC process. As a result,
especially for the application in OLEDs.49–53 In prior studies, trans-PyCNTPA exhibits a small DEST of 0.11 eV and a high FPL of
N-atom containing heterocyclic compounds are commonly 87% in the doped film. The doped OLED based on trans-PyCNTPA
utilized in OLEDs because the sp2 or sp3 hybridization features is achieved with an EQE of 15.5% and a deep-red emission peaking
of N-atoms are beneficial to enhancing the electron and hole at 668 nm. These results reveal that the fine modulation of
injection ability in OLEDs.54 In this case, the oxidation of pyrene molecular structures and rational cis-/trans-isomeric effects all
and further treatment with o-phenylenediamine derivatives could are relative to their performance, which is very important for the
facilely afford the corresponding N-heterocyclic phenanthro[4,5- design of high-efficiency deep-red/NIR TADF materials.
fgh]quinoxaline units, which possess planar molecular structures
to efficiently reduce the vibrational and rotational channels,
suppress the non-radiative transitions to some extent and obtain
Results and discussion
relatively high FPL. In addition, such phenanthro[4,5-fgh]quinoxa-
line units could be expected to have relatively low LUMO energy
levels and favor the application in deep-red/NIR light-emitting
materials as the suitable electron-acceptors. The 1, 3, 6, 8 posi-
tions of pyrene are easily accessible, which provides multiple ways
to modify the molecular structures and fine-tune the materials’
functions.55 When the phenanthro[4,5-fgh]quinoxaline unit is
formed, the substitutions at 1 (6) and 3 (8) positions in various
ways can generate structural isomers conveniently.56,57 The iso-
meric strategy may present a promising method to investigate the
influence of structure–property relationship, comparatively analyze
the isomeric effect and extend the structural diversity of high-
efficiency TADF emitters. The isomeric strategy may present a
promising method to investigate the influence of structure–property
relationship, comparatively analyze the isomeric effect and extend
the structural diversity of high-efficiency TADF emitters.
Synthesis and characterization
The synthetic routes of cis-PyCNTPA, cis-PyPTPA, trans-PyCNTPA,
trans-PyPTPA, trans-PyCNDPA and trans-PyPDPA are illustrated in
Scheme 1. The starting material 1-bromine pyrene was obtained
The isomeric strategy may present a promising method to
investigate the influence of structure–property relationship,
comparatively analyze the isomeric effect and extend the structural
diversity of high-efficiency TADF emitters. Based on these considera-
tion, in this work, four positional isomers, cis-PyCNTPA, cis-PyPTPA,
trans-PyCNTPA and trans-PyPTPA, are developed in which the bulky
triphenylamine (TPA) is employed as the donor owing to its
excellent electron-donating capability, and electron-accepting
phenanthro[4,5-fgh]quinoxaline-10,11-benzene (PQP) and phen-
anthro[4,5-fgh]quinoxaline-10,11-dicarbonitrile (PQCN) are adopted
Scheme 1 Synthetic routes and molecular structures of cis-PyPTPA,
cis-PyCNTPA, trans-PyCNTPA, trans-PyPTPA, trans-PyCNDPA and trans-
PyPDPA. (a) CH2Cl2, ACN, H2O, NaIO4, RuCl3; (b) Tol/H2O, K2CO3,
Pd(PPh3)4, 90 1C; (c) Tol, Pd2(dba)3, t-BuOK, (t-Bu)3PhBF4, 110 1C;
as the acceptor. PQCN is designed by introducing two CN groups (d) AcOH, 125 1C. (e) AcOH, 125 1C.
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via a commercial source. The intermediate 1-bromine-pyrene-2,3-
dione (1) was synthesized by one step oxidation of 1-bromine
pyrene using sodium periodate as an oxidant in the presence of
the RuCl3 catalyst. 1-Bromine-phenanthro[4,5-fgh]quinoxaline-
10,11-benzene (2) and 1-bromine-phenanthro[4,5-fgh]quinoxaline-
10,11-dicarbonitrile (3) were achieved by the similar cyclization
reactions between 1 with o-phenylenediamine and diaminomaleo-
nitrile, respectively. And then, the target compounds cis-PyPTPA
and cis-PyCNTPA were produced through the Suzuki cross-
coupling reaction between (4-(diphenylamino) phenyl) boronic
acid and (2) and (3), respectively. For the synthesis of the other
four compounds with trans-configuration, the different synthesis
routes were adopted. The intermediate N,N-diphenyl-4-(1-pyrenyl)
benzenamine (4) was synthesized through Suzuki cross-coupling
reactions between 1-bromine pyrene and (4-(diphenylamino) phe-
nyl) boronic acid, and N,N-diphenyl-1-pyrenamine (5) was achieved
through the Buchwald-Hartwig cross-coupling reaction between
1-bromine pyrene and diphenylamine. And then, 4 and 5 were
both oxidized to get trans-isomer 9-triphenylamine-pyrene-2,3-
Fig. 1 Optimized ground-state (S0) geometries, HOMO and LUMO dis-
tributions of cis-PyCNTPA, cis-PyPTPA, trans-PyCNTPA, trans-PyCNDPA,
trans-PyPTPA and trans-PyPDPA.
Table 1 Summary of physical properties of cis-PyCNTPA, cis-PyPTPA,
trans-PyCNTPA, trans-PyCNDPA, trans-PyPTPA and trans-PyPDPA
a
b
d
Td
lmax,abs
lmax,PL (nm) HOMO/
DEg
Compound
(1C) (nm) sol
sol/neat film LUMOc (eV) (eV)
cis-PyCNTPA
427 285,347,440 607/685
418 287,351,453 517/565
ꢀ5.23/ꢀ3.56 0.10
dione (6) and 9-diphenylamine-pyrene-2,3-dione (7) with the cis-PyPTPA
catalyst of RuCl3. Finally, the target products were all conveniently
afforded by cyclization reactions with good yields. trans-PyCNTPA
and trans-PyCNDPA were synthesized by the cyclization reaction of trans-PyPDPA
ꢀ5.31/ꢀ3.38 0.34
ꢀ5.25/ꢀ3.60 0.11
ꢀ5.33/ꢀ3.39 0.34
ꢀ5.35/ꢀ3.60 0.08
ꢀ5.37/ꢀ3.38 0.29
trans-PyCNTPA 436 285,364,467 602/690
trans-PyPTPA 435 288,349,446 514/560
trans-PyCNDPA 397 282,365,463 615/715
401 293,354,461 536/590
the diaminomaleonitrile with intermediates 6 and 7, respectively.
a
b
lmax,abs: absorption maximum in toluene.
l
max,PL: emission peak in
c
For the synthesis of trans-PyPTPA and trans-PyPDPA, the similar
procedure was followed but using o-phenylenediamine as the
reactant instead of diaminomaleonitrile. The purity and chemical
structures of the six target molecules were characterized by NMR,
toluene and neat thin films. HOMO/LUMO energy levels measured by
cyclic voltammetry measurement. DEg: calculated from HOMO and
d
LUMO.
mass spectroscopy and elemental analysis. The details of the induce a small overlap of HOMO and LUMO energy level. The
synthesis process are all summarized in the ESI.†
LUMOs were mainly located on the acceptors of PQP and PQCN
segments. For cis-PyCNTPA, trans-PyCNTPA and trans-PyCNDPA
in which the CN was chosen as the co-acceptor, their LUMOs all
Theoretical calculations
To further explore the geometrical and structure–property lied about 0.60 eV lower than those of cis-PyPTPA, trans-PyPTPA
relationship of these isomers, density functional theory (DFT) and trans-PyPDPA, respectively. This could be attributed to the
and time-dependent DFT calculations were performed using stronger electron-withdrawing ability of PQCN compared to
the Gaussian 09 package at the level of B3LYP/6-31G(d,p). Fig. 1 PQP which might lead to a red-shifted emission. To further
displays the optimization of ground state configurations and explore the S1 and T1 states, natural transition orbital (NTO)
the frontier molecular orbital (FMO) distributions. By virtue of analysis was also studied by using time-dependent density
the large steric resistance between TPA/DPA donors and planar functional theory (TD-DFT) calculation (Fig. S7, ESI†). The cis-
PQP and PQCN acceptors in the optimized geometries, a highly PyCNTPA, trans-PyCNTPA and trans-PyCNDPA all possessed
distorted dihedral angle of 58.21 for cis-PyCNTPA, 69.51 for cis- relatively smaller DEST than cis-PyPTPA, trans-PyPTPA and trans-
PyPTPA, 53.81 for trans-PyCNTPA, 60.11 for trans-PyCNDPA, PyPDPA, which might produce more effective RISC processes from
54.71 for trans-PyPTPA and 64.51 for trans-PyPDPA were T1 to S1. The trans-isomers all allowed radiative transitions from
observed, which was helpful for the efficient separation of the S1 to S0, which could be confirmed by the comparatively higher
FMOs and suppression of intermolecular interactions. Com- oscillator strengths of the S1 - S0 transitions than cis-isomers,
pared with the stretching structure molecule of trans-PyCNTPA meaning that higher PL efficiencies could be expected in trans-
and trans-PyPTPA, cis-PyCNTPA and cis-PyPTPA all possessed isomers. Their calculated DEg, HOMO and LUMO energy levels,
more distorted structure and bigger twisted torsion angles and DEST values are all summarized in Table 1 and Table S1 (ESI†).
between the donor and acceptor, which led to fully separated
Thermal and electrochemical properties
FMOs. The HOMOs were dominantly located on the TPA
segments owing to the good electron-donating property in In order to determine the thermal stability of these six compounds,
trans-PyCNTPA and trans-PyPTPA with a little distribution on thermal gravimetric analysis (TGA) and the differential scanning
pyrene. When the donor was changed from TPA to DPA in trans- calorimetry (DSC) were used to test the thermal performance in a
PyCNDPA and trans-PyPDPA, the HOMOs were mainly located nitrogen atmosphere and the results are shown in Fig. 2a and
on the DPA units with sizable distributions on the pyrene Fig. S8 (ESI†). During the whole measurements, these six
group, which indicated that reducing a benzene ring would compounds all displayed good thermal stabilities and the
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Fig. 3 (a) UV-vis absorption spectra in toluene solutions (10ꢀ5 M) and
(b) normalized fluorescence spectra in toluene solutions (10ꢀ5 M) of cis-
PyCNTPA, cis-PyPTPA, trans-PyCNTPA, trans-PyCNDPA, trans-PyPTPA
and trans-PyPDPA.
Fig. 2 TGA traces recorded at a heating rate of 10 1C minꢀ1 (a) and cyclic
voltammograms (b) of cis-PyCNTPA, cis-PyPTPA, trans-PyCNTPA, trans-
PyCNDPA, trans-PyPTPA and trans-PyPDPA.
decomposition temperatures (Td, corresponding to 5% weight
loss) were found in the range of 399 1C to 442 1C. The six
compounds having good thermal stability, during the whole
measurement, exhibited high glass transition temperatures in
the range of 106 1C to 125 1C. This ensured the possibility to
obtain uniform and amorphous films upon thermal evaporation,
which was conducive to device fabrication and operation processes.
The electrochemical properties were investigated via cyclic voltam-
metry (CV) measurement and the HOMO/LUMO energy levels were
obtained by calculating from the respective onset oxidation/
reduction potentials. According to the onsets of the oxidation
curves (relative to the ferrocene/ferrocenium reference) (Fig. 2b),
the oxidation potentials of the six compounds were very close to
each other and their corresponding HOMO energy levels were
all around ꢀ5.30 eV. The reduction potentials were decreased
significantly in cis-PyCNTPA, trans-PyCNTPA and trans-PyCNDPA
and they all showed deeper LUMO levels of ꢀ3.60 eV, which were
all about 0.22 eV lower than respective cis-PyPTPA, trans-PyPTPA
and trans-PyPDPA. The PQCN unit could deepen the LUMO
energy level and effectively contributed to the device performance
because of the relatively facile transfer of electrons. These experi-
mental data demonstrated the same trend as the theoretical
results obtained from the DFT calculations. The CV measure-
ments suggested lower HOMO–LUMO gaps of around 2.15 eV–
2.84 eV, implying that red/NIR emission could be successfully
achieved.
trans-PyCNTPA and trans-PyCNDPA using CN groups as the
co-acceptor showed significant red-shifts with emission maxima
at 607 nm, 602 nm and 615 nm as compared with their counter-
parts of cis-PyPTPA (517 nm), trans-PyPTPA (514 nm) and trans-
PyPDPA (536 nm).
It can also be seen that the two pairs of cis/trans-isomers,
cis-PyCNTPA/trans-PyCNTPA and cis-PyPTPA/trans-PyPTPA all
displayed very identical emission spectra and emission peaks
with only several nanometers’ variations, suggesting that the
molecular configuration had little effect on the emission color.
By changing the polarity of the solvent (Fig. S9 and Table S2,
ESI†), the maximum PL peak of cis-PyCNTPA showed a signifi-
cant red shift from 532 nm in nonpolar hexane to 740 nm in
polar chloroform. The trans-PyCNTPA and trans-PyCNDPA also
showed the same trends with a red-shift from 540 nm in hexane
to 722 nm in chloroform for trans-PyCNTPA, and 554 nm in
hexane to 676 nm in chloroform for trans-PyCNDPA. This is the
general feature for most of the organic D–A type molecules
with substantial CT characters. The solvatochromic effects of
cis-PyPTPA, trans-PyPTPA and trans-PyPDPA were not very
obvious as their PL peaks showed less red-shifts than cis-
PyCNTPA, trans-PyCNTPA and trans-PyCNDPA with increasing
solvent polarity, suggesting the relatively weak electron-
withdrawing ability of the PQP unit.
The effective process of cross-system crossing is the basic
characteristic of TADF material, which requires the molecule to
have enough small DEST. The fluorescence and phosphorescence
Photophysical properties
For the purpose of exploring the photophysical properties of spectra of these compounds were, respectively, investigated in
these compounds, the UV-vis absorption and photoluminescence the dilute toluene at room temperature and at a low temperature
characteristics were further tested. As shown in Fig. 3a, in dilute (77 K), and the experimental S1 and T1 energies were calculated
toluene solution, the absorption spectra of the six compounds from the onsets of the fluorescence and phosphorescence spectra
showed similar absorption profiles with three main absorption (Fig. 4b and Fig. S10, ESI†). The experimental DEST values were
bands. The intense high-energy absorption bands at around calculated to be 0.10 eV for cis-PyCNTPA, 0.11 eV for trans-
280 nm and 330 nm were ascribed to the localized n–p* and PyCNTPA, 0.08 eV for trans-PyCNDPA, 0.34 eV for cis-PyPTPA,
p–p* transitions of the aromatic amines and rigid acceptors PQP 0.34 eV for trans-PyPTPA and 0.29 eV for trans-PyPDPA. In all, the
and PQCN. The structureless long-wavelength broad absorption cis-PyCNTPA, trans-PyCNTPA and trans-PyCNDPA using PQCN
bands in the range of 410–546 nm originated from the ICT as acceptors all manifested much smaller DEST to ensure the
transition from the donor moieties (DPA/TPA) to the acceptor efficient RISC process from triplet excited state to singlet excited
units (PQP/PQCN). They displayed various ICT emissions and state. In order to explore the TADF behavior, the transient PL
structureless PL spectra ranging from the orange-red to deep-red characteristics were also studied. The transient PL decay curves of
region in toluene solutions (Fig. 3b). As expected, cis-PyCNTPA, cis-PyCNTPA, trans-PyCNTPA and trans-PyCNDPA in doped films
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sphere, the PLQYs of the six molecules in the doped films were
also measured. The trans-isomers in the doped films exhibited
a relatively high PLQY of 87% for trans-PyCNTPA, 83% for trans-
PyCNDPA, 81% for trans-PyPTPA and 78% for trans-PyPDPA.
While the cis-conformation molecules possessed lower PLQY of
19% for cis-PyCNTPA and 12% for cis-PyPTPA. This was in the
same trend with the oscillator strengths as found in the
theoretical calculations. The relevant radiative and nonradiative
decay rate constants (kr and knr) were further calculated. In the
excited state dynamics study, the kr of trans-PyCNTPA was
calculated to be 4.10 ꢁ 107 sꢀ1, which was higher than that of
cis-PyCNTPA (0.59 ꢁ 107 sꢀ1). The knrs values of trans-PyCNTPA
and cis-PyCNTPA were comparable. Similarly, trans-PyPTPA
showed a kr of 19.96 ꢁ 107 sꢀ1 which was also higher than that
Fig. 4 (a) Transient PL curves of cis-PyCNTPA, trans-PyCNTPA and trans-
PyCNDPA (1.0 wt%) doped into PMMA hosts (Inset: Prompt components).
(b) Normalized fluorescence and phosphorescence (77 K) spectra of cis-
PyCNTPA, trans-PyCNTPA and trans-PyCNDPA in toluene solutions (10ꢀ5 M).
of cis-PyCNTPA (3.98 ꢁ 107
s
ꢀ1). The knr of trans-PyPTPA was
4.68 ꢁ 107 sꢀ1 which was lower than that of cis-PyPTPA of 29.18 ꢁ
all displayed biexponential decays with nanosecond order prompt
fluorescence decays (tF) and microsecond order delayed fluores-
cence decays (tTADF) (Fig. 4a). However, the remaining compounds
cis-PyPTPA, trans-PyPTPA and trans-PyPDPA only showed single
exponential decays with the prompt tF, illustrating that they were
not emitters with delayed fluorescence (Fig. S11, ESI†). In the
temperature-dependent transient PL spectra, with the temperature
elevating from 100 to 300 K, the delayed component intensities of
cis-PyCNTPA, trans-PyCNTPA and trans-PyCNDPA were all enhanced
with the improving temperature, which were the typical TADF
characteristics (Fig. 5 and Fig. S12, ESI†).
107 ꢀ1. These results indicated that the molecular vibrational
s
modes of excited state of trans-configuration could effectively
hinder the energy loss via nonradiative decay paths and con-
tributed to the higher efficiencies of the trans-isomers (Table 2).
Electroluminescence properties
To explore the EL performance, the non-doped device was first
fabricated with a normal sandwiched configuration of ITO/
HAT-CN (6 nm)/TAPC (45 nm)/TCTA (5 nm)/Emitters (20 nm)/
TmPyPb (40 nm)/LiF (1 nm)/Al (120 nm), in which ITO and Al
served as the anode and cathode, HATCN (1,4,5,8,9,11-hexa-
azatriphenylene-hexanitrile) was used as a hole injection layer,
TAPC (di-(4-(N,N-ditolyl-amino)-phenyl)cyclohexane) and TCTA
(4,4,4-tris(N-carbazolyl)triphenyl-amine) were applied as the
hole transporting layer and exciton blocking layer, TmPyPb
and LiF were used as the electron transporting layer and electron
injecting layer, respectively. The devices exhibited electrolumines-
cence in NIR regions with emission 712 nm for trans-PyCNDPA,
respectively, corresponding to CIE coordinates of (0.65, 0.31), (0.67,
0.31) and (0.67, 0.30). The devices of the remaining three mole-
cules of cis-PyPTPA, trans-PyPTPA and trans-PyPDPA also showed
red emissions with the CIE coordinates of (0.51, 0.47), (0.42, 0.55)
and (0.45, 0.54), respectively. Regrettably, the non-doped OLEDs all
showed quite low EQEs due to the insufficient utilization of triplet
excitons or triplet–triplet annihilations in these non-doped devices
(Table S3, ESI†).
The fluorescence emission spectra of these compounds
doped in the PMMA films were identical to their emissions in
toluene solutions (Fig. S13, ESI†). By using an integrating
The multilayered OLEDs with different device configurations
were further fabricated. OLEDs based on dopants were fabricated
with a normal sandwiched configuration: ITO/HATCN (6 nm)/TAPC
Fig. 5 Transient PL decay curve of the 1.0 wt% trans-PyCNTPA doped
PMMA film at different temperatures.
Table 2 Photophysical properties of cis-PyCNTPA, cis-PyPTPA, trans-PyCNTPA, trans-PyCNDPA, trans-PyPTPA and trans-PyPDPA
Kr (107 sꢀ1
)
knr (107 sꢀ1
)
tF (ns)
tTADF (ms)
kIC (107 sꢀ1
)
kISC (107 sꢀ1
)
kTADF (104 sꢀ1
)
a
Compound
FPL (%)
DEst (eV)
cis-PyCNTPA
cis-PyPTPA
trans-PyCNTPA
trans-PyPTPA
trans-PyCNDPA
trans-PyPDPA
19
12
87
81
83
78
0.10
0.34
0.11
0.34
0.08
0.29
0.59
3.98
4.10
19.96
2.84
21.05
0.25
29.18
0.61
4.68
0.58
5.94
10.30
3.02
10.78
4.06
9.44
3.71
24.36
—
12.67
—
26.02
—
2.52
—
0.61
—
0.58
—
6.60
—
4.56
—
7.17
—
7.80
—
6.81
—
3.19
—
a
FPL: photoluminescence quantum yields of 1 wt% doped PMMA thin films.
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peaks at 640 nm, 669 nm and 668 nm, and Commission Inter-
nationale de L’Eclairage (CIE) coordinates of (0.64, 0.35), (0.63,
0.36) and (0.66, 0.35), respectively, meeting the basic requirements
for deep-red emission (lmax 4 620 nm and CIE coordinates of
(x Z 0.60, y r 0.40)). The EL spectra were all in good accordance
with their PL emission spectra measured in doped films (Fig. 7a),
illustrating that the electroluminescence originated from the pure
emissive layers and energy transfers from the TPBi host to emitters
absolutely occurred in the EL process.
Fig. 6 The device structure and energy level diagram and molecular
structures of the materials employed in the devices.
The devices based on the compounds with PQP as the
acceptor showed poor performance with the maximum EQE of
0.40%, 5.12% and 6.42% for cis-PyPTPA, trans-PyPTPA and trans-
PyPDPA, respectively. As the CN was introduced as the co-acceptor,
the corresponding device performance of cis-PyCNTPA, trans-
PyCNTPA and trans-PyCNDPA demonstrated significant improve-
ment. As a result, the maximum EQE reached 15.5% with the
deep-red emission of 668 nm for trans-PyCNTPA without any
optical out-coupling optimization attributing to the effective
utilization of the triplet excitons in OLEDs, which is a very
descent result among the deep-red TADF OLEDs with an EL
peak wavelength at 660 nm as compared to previously reported
data. In comparison, the OLED based on cis-PyCNTPA showed a
maximum EQE of only 1.56% which was due to its extremely low
PLQY. In addition, the turn-on voltage of the OLED based on
trans-PyCNTPA was significantly lower than that of cis-PyCNTPA.
The cis-PyCNTPA exhibited significantly lower electrolumines-
cence efficiency and photoluminescence efficiency, indicating
that exciton utilization efficiency is lower than trans-PyCNTPA.
The turn-on voltage refers to the driving voltage of the device at
the brightness of 1 cd mꢀ2. Due to the low exciton utilization
efficiency, higher turn-on voltage was needed to produce more
excitons to achieve the brightness of 1 cd mꢀ2, which might
result in higher turn-on voltage for the doped device based on
cis-PyCNTPA. Therefore, the doped device based on cis-PyCNTPA
exhibited much higher turn-on voltage compared with the doped
device based on trans-PyCNTPA. EL characteristics of OLEDs based
on these cis-PyCNTPA, trans-PyCNTPA, trans-PyCNDPA, cis-PyPTPA,
trans-PyPTPA and trans-PyCNDPA are shown in Fig. 7b–c and
Fig. S14a–c (ESI†), and the detailed data are summarized in
Table 3.
(45 nm)/TCTA (5 nm)/TPBi:emitter (20 nm)/TmPyPb (40 nm)/LiF
(1 nm)/Al (120 nm), In the EML, these compounds were doped into
the host 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi)
with 10 wt% concentration. The corresponding molecular struc-
tures, energy level diagrams and the architectures of the devices are
depicted in Fig. 6.
The OLEDs based on trans-PyCNDPA, cis-PyCNTPA and trans-
PyCNTPA exhibited deep-red emission with electroluminescent
Fig. 7 (a) EL spectra of the devices at 100 cd mꢀ2. (b) Luminance curves–
voltage–current density curves of devices based on cis-PyCNTPA, trans-
PyCNTPA and trans-PyCNDPA. (c) EQE–luminance and the EL spectra of
OLEDs based on cis-PyCNTPA, trans-PyCNTPA and trans-PyCNDPA. (d)
The EL spectra of trans-PyCNTPA measured at 4–10 V.
All the devices showed stable spectra under different voltages
(Fig. 7d and Fig. S15, ESI†). These results revealed that the fine
modulation of molecular structure and rational cis-/trans-isomeric
strategy is an effective method to explore the structure–property
Table 3 The EL performances of doped devices based on cis-PyCNTPA, cis-PyPTPA, trans-PyCNTPA, trans-PyCNDPA, trans-PyPTPA and trans-PyPDPA
Lmax (cd mꢀ2
)
CEmax (cd Aꢀ1
)
PEmax (lm Wꢀ1
)
EQEe (%) max/100 cd mꢀ2
EL lmax (nm)
CIEg (x,y)
a
b
c
d
f
Device
Von (V)
cis-PyCNTPA
cis-PyPTPA
trans-PyCNTPA
trans-PyCNDPA
trans-PyPTPA
trans-PyPDPA
5.9
4.5
3.5
4.3
3.8
3.8
555
469
2769
1258
4495
2871
0.95
1.19
10.5
11.2
16.9
20.3
0.51
0.83
9.46
8.21
13.95
16.77
1.56/0.70
0.40/0.22
15.50/3.50
13.90/1.31
5.12/1.87
6.42/1.54
669
536
668
640
544
560
(0.63,0.36)
(0.39,0.56)
(0.66,0.35)
(0.64,0.35)
(0.39,0.56)
(0.45,0.54)
a
b
c
d
Von: turn-on voltage at the luminescence of 1 cd mꢀ2
.
Lmax: maximum luminance. CEmax: maximum current efficiency. PEmax: maximum
power efficiency. EQE: external quantum efficiency of maximum/at 100 cd mꢀ2
commission International de l’Eclairage (CIE) coordinates at 100 cd m
.
EL lmax: EL emission peak of EL spectrum at 100 cd mꢀ2
.
CIE:
e
f
g
ꢀ2
´
.
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Paper
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OLEDs.
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Conclusions
In summary, two pairs of isomers of trans-PyCNTPA/cis-PyCNTPA
and trans-PyPTPA/cis-PyPTPA adopting the same TPA donor and
two different kinds of electron acceptors (PQP and PQCN) with
different donor connecting strategies were designed and synthe-
sized. Two model compounds trans-PyCNDPA and trans-PyPDPA
using DPA as the donor were also obtained. By manipulating the
substitution position of the donor and electron-withdrawing
strength of the acceptor, the emission colors are facilely shifted
from the orange-red (trans-PyPTPA/cis-PyPTPA/trans-PyPDPA) to
deep-red region (trans-PyCNTPA/cis-PyCNTPA/trans-PyCNDPA).
The co-acceptor CN unit is crucial to obtain TADF features. The
cis-PyCNTPA, trans-PyCNTPA and trans-PyCNDPA all manifested
smaller DEST than cis-PyPTPA, trans-PyPTPA and trans-PyPDPA to
ensure the efficient RISC process from the triplet excited state to
the singlet excited state. The experimental data and theoretical
calculations all indicate enhanced FPLs values in trans-isomers,
that is, 87% for trans-PyCNTPA, 83% for trans-PyCNDPA, 81% for
trans-PyPTPA and 78% for trans-PyPDPA, which are remarkably
higher than those of cis-PyCNTPA (19%) and cis-PyPTPA (12%).
The trans-configuration can effectively hinder the energy loss via
nonradiative decay paths and result in higher efficiency. The
TADF-OLED based on trans-PyCNTPA realizes the state-of-the-art
device performance with a high EQE of 15.5% and the electro-
luminescent peak of 668 nm in the deep-red region. These
findings would provide meaningful instructions to achieve highly
efficient red TADF emitters by isomeric modulation for OLED
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This research is supported by the National Natural Science
Foundation of China (91833304, 21774047, and 22075100), the
Open Fund of the State Key Laboratory of Luminescent Materials
and Devices (South China University of Technology, 2021-skllmd)
and the Fundamental Research Funds for the Central Universities.
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